204 EEV. G. HENSLOW — FEETILISATION OF PLANTS. 



but the conclusion, as far as it goes, appears to be this. He tried 

 first of all the Ipomcea purpurea^ and found the relative heights of 

 plants grown from seed of the intercrossed flowers on the same 

 plant, to the self -fertilised, were as 100 to 105. That showed 

 it was better for a flower to be self-fertilised than to be crossed 

 with another flower on the same plant. He cut down both the 

 self-fertilised and crossed plants, i.e. those which had been the 

 result of a seed of flowers fertilised respectively as stated, and 

 the ratio of their weights was as 100 to 124, so that in height 

 and weight it showed that the self-fertilisation was better than 

 the crossing. This was a different result from what was anti- 

 cipated. Botanists had been inclined to take it for granted that 

 it was always beneficial to cross fiowers even on the same plant, 

 as Dean Herbert had surmised. Mr. Darwin only tried five 

 plants. Of Mimulus luteus the height was as 100 to 101, and the 

 weight 100 to 103, the benefit being on the side of the self- 

 fertilised. Digitalis purpurea gave ratios 100 to 94 in height, 

 the weight 100 to 78, showing no slight benefit in crossing the 

 flowers. In another part of his book Mr. Darwin alludes to what 

 some other botanists had found, viz. that no difference occurred 

 with Reseda, Dianthus, or Abut Hon ; but a slight beneficial effect 

 appeared in thus crossing Eschscholtzia, Oncidium and Corydalis cava. 

 A curious fact is noticeable here. All the plants which derived a 

 benefit were naturally more or less self-sterile plants, as stated 

 in his list of self-sterile plants, i.e. which cannot naturally set seed 

 of themselves. Therefore I think we may at least suspect, if not 

 generalise, that the benefit derived from crossing flowers on the 

 same plant may prove to be more or less limited to those naturally 

 self- sterile. 



Kext, we must consider the effects of introducing a new stock. 

 He did this with some half-dozen plants, and then the benefit 

 was something very great indeed. He has given a table (C) in the 

 book, and by tabulating the mean results of it, and taking 100 as the 

 standard for the new stock, we arrive at the following conclusions : 

 Mean of heights of crossed to intercrossed 



,, ,, ,, self- fertilised 



,, fertility ,, intercrossed 



,, ,, ,, self -fertilised 



,, weight ,, intercrossed 



,, ,, ,, self-fertilised 



"Crossed" signifies crossed with anew stock 

 signifies crossed with the same stock. 



You see that in tliis table nearly all these numbers are consider- 

 ably less than 100, showing as a rule that the benefit derived 

 from crossing with a new stock, whether in height, weight, or 

 fertility (estimated by the number of seeds), is immense, liut it is 

 not absolute — there are exceptions, and the number 116 stands out 

 as an exception. Tliat, however, is the mean of only three plants, 

 too few from which to generalise. 



I will enumerate some of the benefits which Mr. Darwin observed ; 



